The present invention generally relates to the area of electronic test equipment. More specifically, the present invention pertains to devices that are capable of measuring electromagnetic or radio frequency (RF) radiated emissions of electronic equipment in the presence of strong ambient signals.
Electronic equipment radiates RF energy (i.e., RF radiated emissions) during normal operation that can interfere with nearby electronics. The trend toward small, low power, high density electronics has made modern electronic devices increasingly more susceptible to such RF interference. For example, turning on a microwave oven near a personal computer may cause the personal computer to lock up, or radio frequency noise from an automobile ignition may introduce static in a nearby AM radio receiver.
To ensure that electronic devices do not produce harmful amounts of radiated RF emissions, government regulators, such as the U.S. Federal Communications Commission (FCC) and the European Union EC Directives, impose radiated emission and immunity regulations on equipment manufactures. Such regulations include ensuring that radiated RF emissions from electronic equipment are below certification levels at specified distances and over specified frequency ranges. Additionally, electronic equipment must be able to function reliably when exposed to certification RF field levels over specified frequency ranges.
A number of commercially available instruments, such as spectrum analyzers and receivers, are used to measure RF emissions. These instruments can be programmed to automatically scan the frequency band of interest, record the detected RF field strength, and compare such RF field strengths with the appropriate certification level. Unfortunately the RF radiation emitted by the electronic device under test can be weaker than ambient field strengths emitted from local transmitters (e.g., signals from TV and radio stations). Commercially available spectrum analyzers and receivers typically cannot differentiate between the RF radiation from the electronic device under test and the much stronger ambient signals. Additionally, the stronger ambient signals will mask the emission signals from the electronic device under test if they both occupy the same frequency.
Several test methods have been developed to isolate and measure RF radiated emissions from electronic devices under test. For example, RF anechoic chamber test methods, remote open area test site test methods, urban open area test site test methods, device power cycling test methods, and ambient cancellation using signal subtraction test methods are commonly used for measuring the RF radiated emissions from electronic devices under test.
The RF anechoic chamber test method measures low level radiated emissions from electronic devices inside a large RF anechoic chamber. The outer walls of the chamber form a shielded room (i.e., metal enclosure) in order to block out the undesired external ambient signals. The inside surfaces of the shielded room are covered with anechoic material that absorbs RF energy. The anechoic material serves to absorb the radiated emissions from the electronic device so that measured signals are not distorted by reflections. Therefore, the radiated emissions measured inside the anechoic chamber will be equivalent to measuring the same emissions outdoors when little or no ambient signals are present. The radiated emissions from the electronic device inside the anechoic chamber are measured using standard field sensors with spectrum analyzers or receivers.
The RF anechoic chamber test method provides a highly accurate and reliable test technique since the RF anechoic chamber suppresses ambient signals by as much as 60 to 100 dB. Furthermore, standard test equipment may be used to measure the ambient signals. Accordingly, the RF anechoic chamber test method is widely used to perform radiated emission certification testing. However, the RF anechoic chamber test method is disfavored due to the necessity to procure and maintain expensive RF anechoic chambers. Specifically, a three meter RF anechoic chamber may cost several hundred thousands of dollars and a ten meter chamber may cost well over a million dollars. Only test laboratories and large manufactures can afford to purchase such chambers in order to use the RF anechoic chamber test method.
As previously mentioned, another test method that is used is the remote open area test site test method wherein an outdoor facility is built in a remote location which is far-removed from sources of ambient signals. The ambient signal strengths at these remote locations are well below the radiated emissions certification levels of the electronic device under test. The radiated emissions of the electronic device can then be measured using standard measurement techniques. The remote open area test site test method produces sufficiently accurate measurements. The ambient signals at the remote test sites are generally 20 to 60 dB lower than those in urban environments. While this test method is not as sensitive as the anechoic chamber test method, it does accurately measure radiated emissions which are near or above certification levels. Furthermore, the remote open area test site test method is widely used to perform radiated emissions certification testing because such method uses standard test equipment. However, the remote open area test site test method is disfavored due to the construction and maintenance costs of the remote outdoor test facility. While the initial cost of a remote test site can be significantly lower than purchasing an anechoic chamber, the maintenance and operational cost of such remote outdoor test facility can be very high. An additional problem is that as cites expand, these remote areas are more difficult to locate. It is very difficult to find a location where the is no TV, radio, cell phones, police radios, and ham radios (to name a few). In areas where this is possible, costs associated with bring personnel and equipment to and from this location is high.
The urban open test site test method is used where access to an anechoic chamber or a remote test facility are not possible. This test method measures radiated emissions of an electronic device in an urban setting. In order to practice the urban open test site test method, an experienced operator with highly sensitive spectrum analyzers or receivers must be present. The experienced operator initially scans the ambient environment to determine the frequencies and field strengths of ambient signals. Typically, radiated emissions deviating from these ambient signals can be readily identified by the operator. In cases where a frequency of a radiated emission is near the frequency of an ambient signal, the experienced operator carefully narrows the resolution bandwidth of the spectrum analyzer in order to separate the two signals. If the frequency of the radiated emission is very close to or at the frequency of the ambient emission, then the urban open test site test method fails to separate and detect the radiated emission.
The urban open test site test method can be conducted in urban outdoor environments. However, this method has limited accuracy for emission frequencies which are not clearly distinct from ambient frequencies. Furthermore, this test method requires highly trained personnel as well as the use of highly sensitive spectrum analyzers. This method cannot be used to certify electronic devices due to the limited accuracy thereof. A major problem with this method is that the bandwidth required to separate these signals is not at the bandwidth specified by the regulatory agencies. Therefore, this measurement technique cannot be used to measure the signal's strength. It can only be used to identify the signals frequency. Another problem with this method is that it can only be used when the emissions signal is not changing frequencies and it cannot be used when the emissions and ambient signals are very close in frequency. However, this method can be used for limited pre-compliance testing and trouble shooting, such as during the process of searching, monitoring, and tracking radiated emission frequencies.
The device power cycling test method requires measuring the ambient signals with the electronic device being off. The ambient signals' power levels and frequencies are recorded. The electronic device is now turned on and another set of measurements is made which include the electronic device emissions and ambient signals. The power level and frequencies are recorded. The two recorded measurements are then overlaid or subtracted from each other with the assumption that the result consists of only measurements from the electronic device's emissions. A major problem with this method is that it does not work if the electronic device's emissions are at the same frequencies, but lower, than the ambient signals. Also, this method does not work if any ambient signals change frequency, which most do. FM radio signals, by definition, are changing frequencies. Another problem is that ambient signals, which change levels or turn on and off, can be incorrectly identified as the device's emissions. A simple example is having a nearby cellular phone off during the ambient only measurements and then having the phone turned on during the device emissions measurements. In this example, the cell phone frequencies would be incorrectly identified as frequencies coming from the electronic device.
The ambient cancellation using subtraction test method attempts to electronically cancel the ambient signals by simultaneously recording RF field measurements at two locations. The first location is near the electronic device being tested and the second location is far therefrom. The second location is sufficiently far away so that it does not detect the weak radiated emissions from the electronic device under test. The much stronger ambient signals are simultaneously recorded at both locations, while the radiated emissions from the electronic device are detected only at the first location. This test method attempts to cancel the undesired ambient signals and isolate the residual radiated emissions from the electronic device by aligning, scaling, and subtracting the signals recorded at both locations.
This test method may be conducted in urban environments. However, it cannot account for differences in two simultaneously received ambient signals caused by multi-path distortion. Accordingly, the ambient signal cancellation is very unreliable. Furthermore, this test method does not take into account the frequency drift and jitter between the two recording receivers thereby making signal alignment very difficult and time consuming. Manual adjustment of alignment, scale, and subtraction for each ambient signal is therefore required at each and every ambient frequency with this test method. This manual alignment process if very time consuming and error prone. The ambient cancellation using subtraction test method additionally fails when the much stronger ambient signals are at the same frequency as the desired radiated emissions of the electronic device since most recording instruments do not have the adequate resolution and sensitivity to isolate the signal of interest by using simple subtraction. Therefore, this test method is not used for compliance or pre-compliance testing because it is highly unreliable.
The present invention addresses the above-mentioned deficiencies in the prior art test methods by electronically sensing and suppressing undesired ambient signals, even when those ambient signals are at the same frequency and stronger than the radiated emissions from the equipment under test. Accordingly, the present invention can yield the performance of a standard spectrum analyzer used inside an enclosed RF anechoic chamber. The present invention does not require expensive RF anechoic chambers, or expensive remote test sites. Additionally, the present invention is automated and does not require highly trained personnel nor the use of highly sensitive spectrum analyzers, nor does it require manual alignment of signals. The present invention accounts for differences in simultaneously received ambient signals caused by multipath distortion thereby resulting in reliable ambient signal suppression. The present invention accounts for ambient signals that change frequency and/or levels, as well as ambient signals that turn on and off. Furthermore, the present invention takes into account the frequency drift and jitter between receivers thereby creating optimal ambient suppression performance. Accordingly, the present invention recovers radiated emissions from an electronic device when such emissions are hidden by much stronger ambient signals. The present invention provides an improvement over the prior art test methods because the present invention provides for a method which can accurately suppress ambient RF signals thereby resulting in measurement of the radiated emissions from the electronic device being tested.